The present invention relates to a photocurable ink composition and an inkjet recording method.
Various ink compositions that are cured by applying activation energy rays such as light have been developed. Such ink compositions are required to exhibit storage stability and curability. Specifically, it is necessary that the ink composition does not polymerize when activation energy rays are not applied during storage, but is polymerized and cured quickly and sufficiently when activation energy rays are applied. However, technology has not yet been developed that can provide an ink composition with both the above characteristics.
For example, JP-A-2006-241194 discloses an ink composition that contains a polymerizable compound, a pigment, a polymer having a nitrogen-containing heterocyclic ring, and a polymerization initiator. In JP-A-2006-241194, a polymer in which a hydroquinone or HALS (hindered amine compound) group is introduced or the like is added to the ink composition as a heat polymerization inhibitor. JP-A-2006-241194 describes that the ink composition exhibits good storage stability and curability upon application of activation energy rays. JP-A-2006-241194 describes that the polymer having a nitrogen-containing heterocyclic ring traps radicals generated in the ink composition due to a dark reaction which suppresses an increase in viscosity (polymerization) during storage.
As described above, a heat polymerization inhibitor may be added to an ink composition in order to suppress generation of radicals due to a dark reaction in order to impart storage stability. However, since a hydroquinone-type heat polymerization inhibitor is consumed during the process of inactivating radicals, the effect does not last for a long time. Since a hindered amine-type heat polymerization inhibitor without an oxidation-reduction cycle is also consumed in the same manner as a hydroquinone-type heat polymerization inhibitor, the effect does not last for a long time. Therefore, a large amount of heat polymerization inhibitor is added to an activation energy ray-curable ink composition. As a result, the effect of inactivating radicals may increase during the photocuring reaction, whereby the curability of the ink composition may be impaired.
Polymerization of a radically polymerizable photocurable ink composition is generally inhibited by oxygen in the air. An amine compound may be added to suppress such a phenomenon. In this case, the amine compound is denatured so that the resulting cured film becomes yellow.
In an example of an attempt to provide an ink composition with both storage stability and curability, JP-A-2003-342499 discloses an activation ray-curable ink composition that contains a polyene compound and a polythiol compound. However, since the photocurable ink composition disclosed in JP-A-2003-342499 is polymerized only via an ene-thiol reaction, the polymerization rate is low. Moreover, the ene-thiol reaction causes a dark reaction which tends to result in gelation tends and a deterioration in storage stability.
There is an increasing for printed articles having a metallic gloss surface formed on the print side. A printed article having a metallic gloss surface has been obtained by a foil stamping printing method that provides a recording medium having a highly flat print side and prints an image on the recording medium using metal foil, a method using vacuum deposition to deposit a metal on a plastic film having a flat print side under vacuum, a method that applies a metallic pigment ink to a recording medium and presses the resulting product, or the like.
A metallic gloss surface with excellent properties may be easily formed by utilizing a metallic pigment for the above-mentioned photocurable ink. However, when adding a metallic pigment to a known photocurable ink, the curability of the coating becomes insufficient so that an excellent metallic gloss surface cannot be obtained. For example, when using a metallic pigment as the pigment in a photocurable ink, light applied to the ink coating is absorbed and reflected by the metallic pigment so that the coating may be cured insufficiently due to a decrease in the quantity of light. When intense light is applied in order to ensure curing of the coating, heat is generated due to light absorbed by the metallic pigment, whereby a recording medium or the like may be damaged. Moreover, such intense light may give rise to safety problems (e.g., light is scattered due to reflection).